The various processes for cracking heavy petroleum cuts, such as catalytic cracking, visbreaking and coking, produce light cuts which are strongly contaminated by different compounds containing sulfur, nitrogen and oxygen. Arsenic is very often detected beside these impurities. The sulfur compounds most often are H.sub.2 S and mercaptans. In the case of catalytic cracking, the presence of CO, CO.sub.2 and COS is also noticed. The nitrogen compounds, which are present in the light cuts, are essentially ammonia or light amines. There is also arsenic itself in compound form with the general formula A.sub.5 R.sub.3, R being for example a hydrocarbon radical such as CH.sub.3 or a hydrogen atom.